The present invention relates to a method in a paper machine for controlling the transverse profile of at least one property of a paper web in which water is removed from the paper web under compression between two forming wires by dewatering and/or forming ribs.
The invention also relates to a device in a paper machine for controlling the transverse profile of at least one property of the paper web which comprises an MB former including a twin-wire zone and an MB unit placed in the zone. In the MB unit, there are two sets of dewatering and/or forming ribs which operate one against the other. At least one of the sets of ribs can be loaded by means of the pressure of a pressure medium against the opposite set of ribs.
Further, the invention relates to an arrangement in a paper machine for regulating the control of the transverse profile of at least one property of the paper web that is being produced in the paper machine. The paper machine comprises a wire part including a twin-wire zone having sets of ribs placed one opposite to the other and operatively pressing against each other. The regulation arrangement includes an arrangement for measuring the grammage profile of the paper web which is placed in connection with, or after, the dryer section of the paper machine and which gives a measurement signal to the process control unit. The process control unit gives a regulation signal to the profiling devices for profiling the profile bar of the slice of the headbox in the paper machine.
In the prior art, the headbox of a paper machine includes a discharge duct which is typically defined from below by a stationary, lower-lip beam and from the opposite side by an upper-lip beam. The upper-lip beam is pivotally fixed in connection with the upper-lip constructions by means of a horizontal articulated joint. At the front edge of the upper-lip beam, there is a profile bar which determines the exact profile of the slice in the transverse direction of the machine. In a manner in itself known, the profile bar includes regulation devices by whose means the profile of the slice is fine-adjusted or "fine tuned". As is known from the prior art, the upper-lip beam is supported from above by means of a number of support arms, typically between 4 and 6. Attempts have been made to arrange this suspension such that the deflection of the upper-lip beam is as small as possible, and the fine adjustment of the profile of the slice has usually been arranged by means of the adjustable profile bar.
In the prior art, headboxes are also known in which the upper-lip beam is supported by means of arms from both of its ends. In addition to the arms, various support and crown-variation arrangements have been used for supporting the headbox, e.g., devices operating by means of hydraulic pressure chambers or hoses.
Attempts have been made to construct the prior art headboxes so that it should be possible to produce a paper homogeneous in respect of its grammage, formation, and strength properties across the entire web width so that a minimum proportion has to be cut off at the edges of the web. In view of achieving these objectives, it is known from the prior art to employ various additional flows or exhaust flows in connection with each side wall at both sides of the discharge duct of the headbox, with respect to which flows reference is made to the assignee's Finnish Patent No. 75,377 (corresponding to the assignee's U.S. Pat. No. 4,687,548, the specification of which is incorporated by reference herein).
It is a particularly important requirement, in particular in the case when producing fine paper, that the principal axes of the directional distribution, i.e., the orientation, in the fiber mesh of the paper coincide with the directions of the main axes of the paper and that the orientation is symmetric in relation to these axes. For example, in the case of copying paper, it is important that the orientations in the upper face and in the lower face of the paper are substantially equal. It is frequently not possible to meet the above requirements sufficiently precisely across the entire width of the web by means of the prior art devices referred to above. Factors that are particularly difficult and that have resulted in complaints have been diagonal curling of a sheet and "tilting" of a pile of forms.
The problems mentioned above have been studied thoroughly by the applicants and assignee. In these studies, it has been possible to establish that the symmetry required from the fiber orientation requires that a transverse velocity of about 1 cm per second to about 3 cm per second is not exceeded at any point of the finished web depending on the jet/wire velocity ratio that is used in the discharge jet. At a velocity higher than this in connection with the web draining process, the main orientation of the fibers is turned so that it diverges by about 5.degree. to about 20.degree. from the machine direction, thereby producing the "tilting" and curling problems. Since a transverse velocity is already produced in the discharge duct along with the attenuation of an uneven main flow profile, principal attention must be directed at the uniformity of the profile of the velocity in the flow direction after the turbulence generator.
One systematic reason for a transverse velocity in the discharge duct is the effect of slowing down the flow in the machine direction by the friction produced by the vertical side walls in the discharge duct. A second systematic effect is the flow or spreading of the pulp slurry in the Fourdrinier wire part in the lateral areas of the wire as a layer of a thickness of about 10 to 30 mm is evened by itself. The prior art additional feeds for the lateral flow are excellently suitable for compensating for these effects (as described in Finnish Patent No. 75,377).
In prior art headboxes, the cross-sectional shape of the discharge duct is measured as of uniform height in the transverse direction. Moreover, it is known from the prior art to operate the crown variation means of the upper-lip beam so that the height of the discharge duct in the transverse direction is as uniform as possible.
It is a known phenomenon that when the paper web is dried, it shrinks in the middle area of the web to a lower extent than in the lateral areas, the shrinkage being usually in the middle about 4% and in the lateral areas about 5% to about 6%. This shrinkage profile produces a corresponding change in the transverse grammage profile of the web so that, owing to the shrinkage, the dry grammage profile of a web whose transverse grammage profile was uniform after the press, has been changed during the drying. Thus, as a result of the change the grammage is slightly higher in both of the lateral areas of the web than in the middle area of the web.
Owing to the drying shrinkage of the web, the grammage-profile regulation automation, which is used commonly in the prior art, sets the profile bar of the headbox more open in the middle area than in the lateral areas. Moreover, since the discharge duct, in the manner known from the prior art, has a rectangular cross-sectional shape in the transverse direction, transverse flows are produced in the discharge jet from the edges toward the center because some of the pulp suspension flowing from the lateral areas of the discharge duct is forced to be shifted towards the middle area of the web. This has a detrimental effect in the profile of the directional angle of the fiber orientation as a so-called S-form. Attempts have been made to control this problem by adjusting the profile of the profile bar to be straighter. However, in such a case it has been necessary to be content with a lower uniformity of the grammage profile.
With respect to the prior art related to the present invention, reference is made additionally to U.S. Pat. No. 4,769,111 assigned to A. Ahlstrom Corporation, to the assignee's Finnish Patent Application No. 885609, and to Finnish Patent Application Nos. 885606 (corresponding to EP 0 371 786) and 885607 assigned to Valmet-Ahlstrom Inc., in which formers marketed under the trade mark "MB-Former" are described. Further, reference is made to the assignee's Finnish Patent Application Nos. 904489, 911281, 913112, and 920228 (corresponding to the assignee's U.S. Pat. No. 5,215,628, U.S. patent application Ser. Nos. 07/850,505, 07/903,603 and 08/006,372, respectively, the specifications of which are incorporated by reference herein), in which various combination concepts of the above MB-former units and of hybrid and gap formers are described.